The self-priming pump is capable of starting to work at the situation that the liquid inlet pipe is not filled with liquid medium (but the must be enough liquid medium in the pump body) and automatically exhausting gas in the liquid inlet pipe. When the pump is started for the first time, there must be enough liquid medium added into the pump body of the self-priming pump; after this, the self-priming pump can start again by the liquid medium remaining in the pump body. The self-priming pump can be divided into two types according to working principle, including internal mixing type and external mixing type. The internal mixing type self-priming pump means that the gas-liquid mixing is carried out near the inlet of the impeller, while the external mixing type self-priming pump means that the gas-liquid mixing is carried out at the outer edge of the impeller. Structure of the external mixing type self-priming pump is as shown in FIG. 1. Before the initial startup, the pump cavity is filled with water at first; after startup, the impeller 1 rotates at a high speed to eliminate liquid medium in the impeller flow channel 2, so as to form negative pressure at the liquid inlet opening of the impeller 1 to suck gas in the liquid inlet pipe 4 into the pump cavity and form gas-liquid mixture with the liquid medium in the pump cavity; and the gas-liquid mixture is discharged to the gas-liquid separation chamber on top of the pump cavity through the impeller flow channel 2 on the impeller; because the exit area of the impeller flow channel 2 becomes big suddenly, the flow velocity decreases suddenly, so the gas separates from the liquid and is discharged from the water outlet pipe 5 on the pump body; and the liquid medium sinks and flows back to the outer edge of the impeller 1 through the backflow gap 3 because it is heavy, and it continues to mix with gas sucked in from the impeller flow channel 2; all gas in the liquid inlet pipe 4 can be gradually discharged after repeated circulation to make the liquid medium enter into the pump cavity and finish the self-priming process.
However, because there is a backflow gap 3 in the self-priming pump structure, the remaining medium keeps circulating under action of the pressure, which causes big loss in volume efficiency. The test shows that the loss is about 8%, which lowers down the efficiency of the self-priming pump seriously. Besides, the self-priming pump cannot reach the normal flow and pump head, and energy consumption is increased.